[site/docs] Add missing meta headers
[site/docs] Add missing meta headers

Adding the missing viewport header that will make the page layout nicely
on mobile, as well as the social metadata used when sharing the pages.

Signed-off-by: Garret Kelly <d5e09c72d3e525ee6ce60f794290ad8eb687c905@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan